1. Field of the Invention
The present invention relates to a voltage regulator including an overcurrent protection circuit.
2. Description of the Related Art
Description is made of a conventional voltage regulator. FIG. 9 is a diagram illustrating the conventional voltage regulator.
The conventional voltage regulator includes a ground terminal 100, a power supply terminal 101, an output terminal 102, a reference voltage circuit 103, a differential amplifier circuit 104, an output transistor 105, a voltage dividing circuit 106, and an overcurrent protection circuit 107.
Description is made of an operation of the conventional voltage regulator.
When an output voltage Vout of the output terminal 102 is higher than a predetermined voltage, that is, when a divided voltage Vfb of the voltage dividing circuit 106 is higher than a reference voltage Vref, an output signal of the differential amplifier circuit 104 becomes higher. A gate voltage of the output transistor 105 increases, and hence the output transistor 105 is gradually turned OFF and the output voltage Vout decreases. On the other hand, when the output voltage Vout is lower than the predetermined voltage, the output voltage Vout increases in the same manner as described above. In other words, the output voltage Vout of the voltage regulator is maintained to a constant predetermined voltage.
When the output voltage Vout of the voltage regulator decreases due to an increase in load, an output current Iout increases to be a maximum output current Im. Then, in accordance with the maximum output current Im, a larger current flows through a sense transistor 121 which is current-mirror-connected to the output transistor 105. At this time, a voltage generated across a resistor 154 increases to gradually turn ON an NMOS transistor 123, and a voltage generated across a resistor 153 increases. Then, a PMOS transistor 124 is gradually turned ON, and a gate-source voltage of the output transistor 105 decreases to gradually turn OFF the output transistor 105. Accordingly, the output current Iout does not exceed the maximum output current Im but is fixed to the maximum output current Im, and hence the output voltage Vout decreases. In this case, due to the voltage generated across the resistor 154, the gate-source voltage of the output transistor 105 decreases to gradually turn OFF the output transistor 105, and the output current Iout is fixed to the maximum output current Im. Therefore, the maximum output current Im is determined by a resistance of the resistor 154 and a threshold of the transistor 123 (see Japanese Patent Application Laid-open No. 2005-293067).
In order to set an accurate maximum output current Im, it is necessary to adjust the resistance of the resistor 154 and the threshold of the transistor 123 accurately. For the adjustment, trimming is performed after evaluation of characteristics of the resistor 154 and the transistor 123. The evaluation is performed on alternative elements having the same characteristics as those of the resistor 154 and the transistor 123.
FIG. 10 is a diagram illustrating a conventional voltage regulator including a test circuit. The conventional voltage regulator including the test circuit further includes a voltage detector 111, a first switch 191, a second switch 192, and an alternative element 112 under evaluation.
When an output of the voltage dividing circuit 106 is input to the voltage detector 111, the first switch 191 is controlled by an output of the voltage detector 111. When the first switch 191 is short-circuited, a current flows through the alternative element 112 under evaluation from the output terminal 102. When the second switch 192, which is controlled by the output of the voltage detector 111, is short-circuited, a PMOS transistor 129 is gradually turned OFF, and no current flows through an internal circuit element 113 from the output terminal 102. Accordingly, with the use of the configuration of FIG. 10, the electrical characteristics of the alternative element 112 under evaluation can be evaluated accurately (see Japanese Patent Application Laid-open No. 2008-140113).
In the conventional technology, however, in order to perform overcurrent protection trimming to set the maximum output current Im of the voltage regulator accurately, it is necessary to prepare a special test circuit for evaluating the element that determines the maximum output current Im. The test circuit becomes unnecessary when the voltage regulator functions as a product. Accordingly, the presence of the test circuit leads to a larger chip area of a voltage regulator IC. As the chip area increases, the number of chips per wafer is reduced, which is disadvantageous in terms of cost. In addition, the presence of a test step of evaluating the electrical characteristics of the alternative element under evaluation leads to a higher manufacturing cost of the IC, which is disadvantageous in terms of cost.